This invention relates to an integrated system comprising an oxygen selective ion transport membrane cooperating with a silicon oxidation furnace to provide ultra-high purity oxygen to the furnace as a reactant for producing a highly pure silicon dioxide coating. This integrated system affords a particular advantage inasmuch as the heat source employed for efficient operation of the silicon oxidation furnace is also suitably employed to provide a desired elevated temperature for efficient operation of the membrane, thereby insuring production of the desired high-purity oxygen permeate.
Silicon dioxide is a key component for the manufacturing of semiconductors. Conventional processes for oxidizing silicon to form silicon dioxide typically employ oxygen-containing reactants, such as oxygen, air, steam, or a combination thereof, in furnaces that operate at high temperatures, such as from about 900xc2x0 C. to about 1,000xc2x0 C. The quality of the silicon dioxide coatings produced by the oxidation process is adversely affected by the presence of impurities in the gas phase of the reactor, and the semiconductor industry demands highly pure coatings. Accordingly, oxygen itself, in very pure form, is the preferred reactant for providing thin-layer films of silicon dioxide, typically having a film thickness of from 5 to 10 nanometers (nm). More particularly, ultra-high purity (so-called xe2x80x9cUHPxe2x80x9d) oxygen containing contaminants in a total amount not exceeding 100 parts per billion is desirably employed to oxidize the silicon to form silicon dioxide having the desired coating purity.
In order to achieve the required high level of silicon dioxide purity, contaminants such as argon (Ar) and krypton (Kr), as well as hydrocarbons, nitrogen, and other contaminants that tend to adversely affect the quality and/or growth of the coating, are removed from the oxygen reactant before effecting silicon oxidation. Several methods are known for xe2x80x9coff-sitexe2x80x9d production of the desired UHP oxygen. After off-site production, the UHP oxygen is then suitably shipped to the site of the semiconductor plant for use in the oxidation furnace. Heretofore, off-site UHP oxygen production was typically done by cryogenic distillation of air to form so-called xe2x80x9chigh purityxe2x80x9d (also referred to as xe2x80x9cHPxe2x80x9d) oxygen, containing no more than 0.5% by weight of impurities, followed by further refinement of the HP oxygen to produce the desired UHP oxygen. This process is expensive, and the resulting UHP oxygen must then be shipped to the site of the silicon dioxide plant for use as desired. Further, if this expensive methodology is used xe2x80x9con-sitexe2x80x9d to purify the bulk oxygen supply to a microelectronics plant, the cost becomes prohibitively expensive. Moreover, such xe2x80x9cbulkxe2x80x9d purification methodology typically results in wasted oxygen purification efforts when used to produce oxygen reactant for portions of the plant that do not require such high purity, UHP oxygen, since the requirements for UHP material are typically localized within the plant.
Another method that is useful for producing contaminant free oxygen for use in industrial application involves the utilization of an oxygen selective ion transport (ceramic) membrane. These ceramic membranes are capable of selectively transporting oxygen ions across the membrane, and are used to separate pure oxygen from gas mixtures in a variety of industrial applications, although not heretofore in combination with a silicon oxidation furnace.
Ceramic membranes formed from solid electrolytes and mixed conducting oxides typically exhibit the property of oxygen selectivity. xe2x80x9cOxygen selectivityxe2x80x9d means that only oxygen ions are transported across the membrane, while other elements and ions are excluded. These mixed conductor ceramic membranes (also referred to as xe2x80x9cionic/mixed conductor membranesxe2x80x9d) are known to be generally useful for purifying oxygen, although not heretofore in combination with a silicon oxidation furnace.
By way of illustration, U.S. Pat. No. 5,306,411 (to Mazanec et al) discloses that ceramic membranes are suitably employed to produce oxygen for oxidation reactors. Additionally, U.S. Pat. No. 5,580,497 (to Balachandan et al) teaches the use of dense ceramic ion conductors that are suitably used to produce high purity oxygen. Further, U.S. Pat. No. 5,380,467 (to Ching-Yu Lin) teaches that ionic conductors are suitably used to produce high purity oxygen in an pressure-driven mode, whereas International Patent Application WO 95/27810 (to Renlund et al) describes such production utilizing the ionic conductor in an electrically-driven mode. None of these patents disclose the use o f a ceramic membrane for oxygen purification in connection with a silicon oxidation plant.
In view of the inconvenience and expense associated with known methods for supplying UHP oxygen to silicon oxidation furnaces, the microelectronic components manufacturing community has a need for a system for cost effectively producing on-site UHP oxygen within, or in close proximity to, the silicon oxidation furnace itself. The present invention provides an answer to that need.
It is therefore an object of the invention to provide a process for integrating a mixed conductor ceramic membrane into the operation of a silicon oxidation furnace for oxidizing silicon to form silicon dioxide, and to take advantage of the energy efficiencies associated therewith.
It is a further object of this invention to provide a process that will provide UHP oxygen to specific site(s) within a silicon oxidation furnace requiring such UHP oxygen, while avoiding the requirement to ship UHP oxygen from outside the plant or use it throughout the factory environment.
Yet another object of this invention is to integrate the operation of a silicon oxidation furnace, used to form silicon dioxide and typically operating at temperatures in excess of 900xc2x0 C., to provide heat for an integrated selective oxygen transport membrane cell. This facilitates using the same heat source that is required for the operation of the oxidation furnace to provide the high temperatures required for the proper functioning of the membrane.
In one aspect, the present invention relates to an integrated system for producing high purity silicon dioxide comprising:
a) a source of an oxygen-containing feed gas containing at least one impurity,
b) an oxygen transport membrane cell containing an oxygen-selective transport membrane that has a cathode side and an opposing anode side, said membrane being at an elevated temperature effective for separation of oxygen in said feed gas from said impurity by transporting oxygen ions from said oxygen-containing feed gas through said membrane to said anode to form a purified oxygen permeate on said anode side, while retaining an oxygen-depleted, impurity-containing retentate on said cathode side,
c) a passageway from said source (a) to the cathode side of said membrane cell,
d) a silicon source, (commonly a silicon wafer), and
e) a silicon oxidation furnace, in communication with said anode side of said membrane cell, for reaction of said purified oxygen permeate with silicon from said silicon source, at an elevated reaction temperature effective for said reaction, in order to produce said high purity silicon dioxide.
In another aspect, the present invention relates to a process for producing a high purity silicon dioxide coating on a substrate comprising contacting a surface of the substrate with silicon dioxide produced using the above-described integrated system.
In yet another aspect, the present invention relates to a method for preparing pure silicon dioxide comprising the steps of:
A) feeding an oxygen-containing feed gas into a cathode side of an oxygen transport membrane cell containing said cathode side and an anode side, with an oxygen transport membrane therebetween,
B) selectively transporting oxygen ions from said oxygen-containing feed gas from said cathode side through said membrane to said anode side to provide a purified oxygen permeate,
C) reacting said purified oxygen permeate with silicon in a silicon oxidation furnace to form high purity silicon dioxide.
In still another aspect, the present invention relates to an integrated system for the delivery of high purity oxygen to a silicon oxidation furnace and for using the high purity oxygen to prepare high purity silicon dioxide comprising:
a) an oxygen transport membrane cell containing an oxygen selective ion transport membrane that has a cathode side and an opposing anode side and is at a temperature effective for the transport of permeate oxygen from said cathode side to said anode side,
b) an oxygen-containing feed gas contacting said cathode side wherein oxygen ions from said feed gas are transported to said anode side to provide an oxygen permeate, and an oxygen-depleted retentate is retained as an effluent stream on said cathode side,
c) reacting a reaction mixture comprising said oxygen permeate and silicon from a silicon source in a silicon oxidation furnace heated to an elevated temperature sufficient to react the reaction mixture, thereby forming said high purity silicon dioxide.
These and other aspects will become apparent upon reading the detailed description of the invention.